The repair of load-carrying skeletal members such as fractured bones, damaged or diseased joints, amputations, resections for malignancy or disease, and painful, malformed or otherwise mechanically affected skeletal components have hitherto been treated by the surgical techniques discussed below.
One most natural type of repair is autologous bone graft. Its use, however, exhibits at least one obvious, major deterrent, namely that of necessitating the complication of opening a second surgical site or bony sacrifice to provide the graft material. In addition, this type of repair requires special skill and improvisation on the part of the surgeon in fashioning the repair at the operating table within the time alloted for the surgery.
Various types of rigid and semi-rigid structurally splinted or stemmed devices have been used as bone replacement or as reinforcements or attachments to skeletal structure with varying degrees of success. Generally their use is initially very beneficial but over a period of years and months their efficacy often deteriorates because of loosening of the body attachment. It is believed that this effect results from high localized pressure loads imposed upon the bone by the hard-surfaced prosthetic material which tends to pinch off fine blood vessels and crush adjacent tissue, producing resorption of bone and necrotic degeneration in the affected zone. This effect is often noted only after the patient has been partially rehabilitated and attempts to put the affected limb or member into normal vigorous use.
At the forefront of current practice in this art are several techniques generally recognized as significant advancements in improving the biological compatibility of the prosthesis-to-bone interface and they are discussed below.
A. Bone cement.
This refers to an embedment system in which the metal stem of a prosthesis is cemented into intimate contact with porous hollow bone structure. A major advantage of this system is more uniform distribution of mechanical loads, elimination of relative motion between prosthesis and bone, and the achievement of much lower load per unit area (psi loading) than in devices of the earlier art, such lower loading more closely approaching the normal bone loading of undamaged natural skeletal structure. Problems associated with this technique include toxicity of the cement, necrosis of the adjacent bony layer due to heat of polymerization, incomplete filling of the cavity in the bone, and absence of resiliency.
B. Porous ceramic devices and porous ceramic coated metal stems.
This type of construction allows for a thin layer of tissue ingrowth into the pores and results in a very satisfactory type of biological interface where the brittle ceramics can be tolerated. Initial ingrowth of the patient's tissue into the prosthesis is, of necessity, fibrous and does not tend to develop calcium-rich bone until months or years postoperatively. Such ingrowth can be encouraged in the porous ceramic device by carefully controlled sandblasting to induce surface roughness while avoiding deep porosity with weakening of the ceramic structure. Since the tissue penetration is minimal, the joint strength is dependent on shear strength and resistance to cleavage in the thin fibrous attachment zone. Failure in either mode is usually complete and results in failure of the device. In addition, a shock mitigating resilient layer is absent.
C. Velour fiber coated metal stems.
The open literature reports over two years of animal testing involving partial limb replacement in amputations in which metal implants were velour-covered over a limited area at the distal end to obtain skin closure through tissue ingrowth. These tests did not make use of either full fabric jacketing or resilient cushioning. Therefore, the life of such implant does not usually exceed, and is often less than, six months. It is to be expected in such implants that even if it were fully fabric jacketed but without resiliency, in the longer term, the problems of rigid metal-to-bone interfacing would again be encountered after true bony fixation was obtained.
The above disadvantages are overcome and other advantages are also obtained by the present invention as will become apparent from the description below. This invention provides a strong, reinforced prosthesis which has a resilient, cushioning coating structure enclosing the reinforcement means, and a fibrous overlayer to encourage ingrowth of the patient's tissue and firm bonding of the prosthesis to the desired elements of the patient's or host structure. Thus, the device of this invention avoids the rigid or abrasive metal-to-bone facing, the brittleness of ceramic surfaces, has been observed to insert readily with minimal infection occurrences, and has exhibited good longevity in use in a patient. In addition, bonding of the components to each other is so efficiently and firmly effected in the present invention as to provide a reliable, load-bearing, high-strength transfer of loading from the reinforcing means across the elastomer jacket to the host bone.